mechanical elements
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Mechanical Elements
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Outline:
•Mechanical properties of materials•Machine elements: 1) Motors (types, selection considerations) 2) Power transmission devices (gears, belt, pulley, chain, friction drive) 3) Miscellaneous: Bearings(types, selection issues, catalogs), springs, pins, retaining rings… 4) Other motion generation devices (linkage mechanisms, cams) 5) Joining methods (welding, brazing, soldering, bolts, screws, rivet, …)
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Mechanical properties of materials:
Behavior of materials in response to mechanical loads. Useful in material selection.
Most basic property: stress-strain relation
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Stress:
Strain :
)(
)(
Aarea
Fload
)(
)(
0lngthoriginalle
lgelengthchan
Strength of a material
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Maximum stress that can be applied to material before failure.
Types: - Tensile Strength - Compressive strength - Shear strength
The definition of failure varies by the type of material and design method: Brittle (concrete, cast iron, glass,…) Ductile (gold, Aluminum, copper, steel, …)
Stress-Strain relation
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The stress caused by load must not exceed the failure stress.
Always consider a Factor of Safety.
FS= σall /σdesign
σall = Allowable Stressσdesign = Design Stress
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Stress-strain curve, Aluminum[1] Stress-strain curve for a brittle material [1]
Standards
• SAE (Society of Automotive Engineers)• AISI (American Iron and Steel Institute)• ASTM (American Society for testing and Materials)• ANSI (American National Standard Institute)
AISI-SAE designation for carbon and alloy steel
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Mild steel 0.1% - .25% Carbon e.g., AISI 1018 Medium carbon steel .25% - .45% e.g., AISI 1040
Machine elements
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•Motor•Gear•Belt, pulley•Chain, sprocket•Universal joint•Friction drive•Cam-follower •Mechanisms (linkages)
•Bearing•Joining methods (welding, brazing, rivets, bolts, screws, etc)
Motor types
• DC motors
• Stepper motors
• Gearhead motors
• AC motors
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a) Permanent magnet (brushless)
b) DC Shunt motorc) DC series motor
Characteristics
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DC motor
a) Speed proportionate to voltage applied
b) Suitable current/torque and speed/torque curve properties
c) Easy to control
d) Position/Speed Servo
e) No brush noise, durable, clean (brushless)
Characteristics
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Stepper motor
• Can rotate in both directions
• Moves in precise angular increments (steps)
• Sustain a holding torque at zero speed
• Easy to control
Comparison of stepper and DC permanent-magnet motors
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Selection factors
When selecting a motor consider these issues:
• Speed range• Torque-speed variations• Reversibility• Required power• Load inertia
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•Starting torque•Size and weight restrictions•Price •Maintenance
Selection factors (cont’d)
Consider these questions when selecting a motor:
1. Will the motor start under load?
2. What is the maximum speed the motor can produce?
3. How much power does the load require?
4. Is the load to be driven at constant speed?
5. Is transmission gearbox required?
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Useful information sources
Experts Manufacturer specification
sheets Product catalog Design handbooks Motor nameplate Web
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Gearhead motors
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Force ratio for gear trains
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RRR
rWrrF
W=Load force F= balancing force
rFFR 1
R
r
Gear train speed ratio
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driver
driven
ddd
DDDspeedratio
321
321
driven
drive
NN
NN
n
n
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1
4
n= speedN= number of teeth
Different kinds of power transmission or motion control capability of gears:
a) Changing rotational speed.
b) Changing rotational direction.
c) Multiplying or dividing torque.
d) Converting rotational to linear motion.
e) Offsetting or changing the location of rotating motion.
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Motion and torque transmission examples ([5])
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•Right angle crossing shafts•Self locking•High friction and wear•High speed reduction
•Right angle I/O torque•Smooth tooth interaction•Low noise
Rotary-Linear power transmission
Bearing types
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Ball bearing
Tapered roller bearing
Needle rollerbearing
Thrust bearing
Important factors in bearing selection• Loads (radial, axial)• Operating speed• Size and weight
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A quick way of bearing selection
Information sources:• Experts • Manufacturer’s catalog (SKF, TIMKEN, FAG,…)• Design handbook
SKF online interactive catalog (www.skf.com)
SKF interactive bearing selection example page
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Other ways of motion transmission: Linkage mechanisms
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Intermittent rotary motion[6]
Rotary to back and forth motion[7]
Alternative mechanism design
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Flexible transmission
• Chain-sprocket• Belt-pulley
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Ref[8, 9]
Ref[10]
Joining methods• Welding• Brazing• soldering
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Joining methods
• Welding: (melting both material and filler, generally used for welding ferrous materials)
• Brazing: (melting nonferrous metal, brass or bronze, as filler to join base materials by capillary action)
• Soldering: ( same as brazing but at lower temperatures)
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Joining method
Joint strength
temperature Distortion Aesthetics
Soldering Poor up to 400°C None Good
Brazing Good 800-1000 °C Minimal Excellent
Welding Excellent above1500°C Likely Fair
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e.g., Brazing with Bronze alloy as filler with 870-980°C for joining mild steel with melting temperature of 1600°C[1].
Welding types
Arc welding: An electric arc between material and filler melts them at the joining point.
Gas welding (oxyacetylene): Widely used for welding pipes and tubes and repair work
Resistance welding:
Generating heat by passing current through resistance caused by
joining metals. (widely used in automotive industry)
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Arc Welding: Setup configuration [11]
1- power supply2- electrode holder3- workpiece4- work clamp
Welding position
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1- workpiece 2- work clamp3- slag4- electrode5- electrode holding position
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Electrode position in groove welding
Electrode position in fillet welding
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1- stringer bead (steady movement along seam)
2- weave bead (side to side movement along seam)
3- Weave pattern
Single stringer bead for narrow grooves
Weave bead for wide groove or multiple stringer bead
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Properties of a good welding
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Excessive spatter
Cause: 1-Amperage too high2-Arc length too long
Correction:1- Decrease amperage or select larger electrode2- Reduce Arc length
Porosity
Cause: 1-Dirty workpiece2-Arc length too long3-Damp electrode
Correction:1-Remove all grease, damp, oil, dust,..., from work surface 2-Reduce length3-Use dry electrode
Lack of penetration
Causes: 1-Improper weld technique 2-insufficient heat input
Correction:1-Reduce welding speed 2-Increase amperage, use larger electrode
Troubleshooting
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Burn through
Cause:Excessive heat
Correction:Decrease amperageuse smaller electrodeIncrease travel speed
Wavy bead
Cause:Unsteady hand
Correction:Use two hands practice
Distortion
Cause:Excessive heat
Correction:Use clamp to hold base materialUse lower amperageMake tack weld along the edgeWeld in small segments, allow cooling between welds.
Troubleshooting
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Example of good and bad welds
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